This invention relates to the control of stepper motors in general and more particularly to an improved control arrangement for such motors utilizing a digital frequency smoother.
Stepper motors or stepping motors are being used with increasing frequency as the drive elements of movable machine parts such as in machine tool controls and in office machines. However, groups of control pulses for the stepping motors are often furnished thereto by control systems at an erratic or irregular intervals. Since stepping motors can handle pulse jumps only within given limits, it therefore becomes necessary to smooth these frequency jumps. This problem can be solved, without using analog elements through the use of what is known as digital frequency smoothing. Such digital frequency smoothing is disclosed in a paper entitled "Digital Solutions of Control Problems in Numerical Path Controls", by Gose, published in "Regelungstechnische Praxis und Prozess-Rechentechnik" 1973, No. 7, pages 167 ff, in Sections 4 and 5, pages 169-170.
When a series of m pulses at constant frequency f.sub.E is supplied to the input of the digital frequency smoother, there will be provided at the output of the frequency smoother a series of m pulses but with a variation in frequency. Starting with an initial value the output frequency increases according to an exponential function until it reaches, with a sufficiently large number of pulses, the value of the input frequency and then again decreases exponentially once the input pulses are terminated. It will be recognized that for a small number of input pulses equilibrium is not reached. However, the exponential raising and falling of the output frequency still remains.
A property of such frequency smoothing is that the output pulses are considerably delayed in relation to the input pulses. Even the first output pulse lags behind the first input pulse. Furthermore, due to the exponential decay of the frequency there is an extremely long time delay between the next to the last and the last output pulse in proportion to the total time sequence of the pulse series.
In very many applications, e.g. in machine tool controls, these two properties do not play an important role. That is to say the delays can be easily tolerated. However, in cases where, in addition to a requirement for frequency smoothing and pulse preservation, the drive unit equipped with the stepping motor must meet special requirements as to the time to reach an exact positioning, such a control is inadequate. An example of this type of apparatus is an electric typewriter in which the stepping motor is used for positioning the carriage carrying the typing head, or the typing head itself. In order to be able to reach a desired typing frequency of, for example 20 Hz (50 ms per character) the carriage or head movement must be executed in 30 ms.
It would be possible to condense the time sequence of the output pulses being provided from the digital smoothing apparatus by increasing its computing frequency. However, such an increase would also result in the time constant of the frequency increase being shortened and the starting frequency of motor increased. However, since the stepping motor can be operated only at a limited start and/or stop frequency, the motor cannot follow the pulse pattern that is required in typewriter technology and conventional digital frequency smoothing cannot be readily used to fulfill all requirements.
In view of these problems the need for an improved system which offers the advantages of digital frequency smoothing but eliminates the disadvantages of delays and a long settling time becomes evident.